The field of the invention relates to control systems for maintaining engine air/fuel operation in response to an exhaust gas oxygen sensor.
Feedback control systems responsive to exhaust gas oxygen sensors which attempt to maintain engine air/fuel ratio near the peak efficiency window of a catalytic converter are well known, The sensor output is typically compared to a reference value which under ideal conditions is at the approximate midpoint in expected peak-to-peak excursion of the sensor output, A two-state signal is thereby generated which indicates when engine air/fuel operation is either rich or lean of a predetermined air/fuel ratio such as stoichiometry. In an attempt to compensate for fluctuations in the sensor output due to deterioration, contamination of the electrodes, or low operating temperature, an approach was disclosed in U.S. Pat. No. 4,170,965 to time average the sensor output through an RC filter, and use the time averaged value as the reference value.
The inventors herein have recognized several problems with the above approach. Using a time averaged output of the EGO sensor as the comparison reference will not always result in alignment of the reference with the midpoint in peak-to-peak excursion of the EGO sensor output. Because such a value is an average of past history, it will not track rapid shifts in the sensor output. Such shifts may occur, for example, when the sensor heater has not stabilized. Sensor temperature is then dependent on engine operating conditions so that sudden temperature changes may occur resulting in abrupt shifts of the sensor output in either a lean or a rich direction. Shifts in the sensor output may also be caused by changes in exhaust pressure. For these and other reasons, the switch point in the sensor output may not be in perfect alignment with the peak efficiency operating window of the catalytic converter.